Hybridoma cells contaminated with large round cells!

Lately we have been growing mouse hybridoma cell lines and frequently observe hybridoma cells contaminated with some large and round cells (almost double the size of hybridoma cells). Eralier we thought these could be the cells which are about to divide or the ones about to die by apoptosis but these seem to increase in number. Can anybody suggest what these possibly could be? Does the culture require another round of single cell cloning? Please help